WO-A1-2004/072449, the disclosure of which is incorporated herein by reference in its entirety, describes various forms of a supercharger for use with an automotive engine. These superchargers differ from a conventional supercharger in being operable to augment rotary power from the engine crankshaft with rotary power from one or more electric motors housed in a body of the supercharger.
For example, at least one of the superchargers described in WO-A1-2004/072449 includes an input shaft for coupling by a transmission belt to the crankshaft of the engine, and also includes an output shaft to which is fitted an air impellor. The supercharger further includes an epicyclic gear train, and first and second motor-generator electrical machines. The arrangement is such that the input shaft is coupled to the annulus of the epicyclic gear train and to the rotor of one of the electrical machines; the output shaft is coupled to the sun wheel of the epicyclic gear train; and the carrier of the epicyclic gear train is coupled to the other electrical machine.
This supercharger is advantageous in that it is operable to vary the pressure of air in the inlet manifold of the engine independently of the engine speed, and in an efficient and cost-effective manner. By varying the air pressure independently of engine speed, the supercharger can be very responsive to changes in load.
One known application of such superchargers is use together with one or more turbochargers to form a boosting system for an automobile engine. In one such known arrangement, the outlet of the compressor of the turbocharger is coupled to the inlet of the compressor of the supercharger; in other words, the turbocharger compressor is positioned upstream of, and in series with, the compressor of the supercharger.
This first arrangement is shown in FIG. 1 of the drawings. In another such known arrangement, the positions of the two are swapped: the turbocharger compressor is positioned downstream of, and again in series with, the compressor of the supercharger. This other arrangement is shown in FIG. 2 of the drawings.
These known arrangements increase the performance envelope and response rate of the resulting boosting system, thereby enabling higher engine output across a wider speed range and hence improved driving characteristics. In each of these arrangements, each turbocharger may be provided with one or more known means of controlling boost pressure, for example: exhaust wastegate, variable inlet guide vanes on turbine and/or variable turbine area.
In the first arrangement, in which the turbocharger is position upstream of the supercharger, it is typical to have a bypass around the supercharger compressor, that is from the inlet of the supercharger compressor (which essentially the same as the outlet of the turbocharger compressor), to the outlet of the turbocharger compressor. This is so that, at high airflow rates, all or part of the flow can avoid the supercharger compressor. This first arrangement is shown in FIG. 1 of the drawings.
In the second arrangement, in which the supercharger is positioned upstream of the turbocharger, it is also typical to have a bypass around the supercharger compressor, that is from the inlet of the supercharger compressor to the outlet of the supercharger compressor (which is the same as the inlet of the turbocharger compressor. Again, this is so that at high airflow rates all or part of the flow can avoid the supercharger compressor. This second arrangement is shown in FIG. 2 of the drawings.
With respect to FIG. 1, position 1 refers to a point or number of points upstream of the turbocharger compressor(s), position 2 refers to a point or number of points between the turbocharger compressor(s) and the supercharger compressor and position 3 refers to a point or number of points downstream of the supercharger compressor.
With respect to FIG. 2, position 1 is a point or number of points upstream of the supercharger compressor, position 2 is a point or number of points between the supercharger compressor and the turbocharger compressor(s) and position 3 is a point or number of points downstream of the turbocharger compressor(s).
Many turbocharged automotive engines have exhaust gas recirculation (EGR) systems which are commonly used to reduce NOx emissions or improve fuel consumption. These systems take exhaust gas from the exhaust system of an engine and feed it into the intake system of the engine. Various EGR systems are known. A first one of these is the so-called “low pressure” system where exhaust gas is taken from a point downstream of the turbocharger turbine (and usually after a particulate filter) and fed (usually through a cooler and a control valve) into the intake system upstream of the turbocharger compressor.
Another known EGR system is the so called “high pressure” system where exhaust gas is taken from a point upstream of the turbocharger turbine and fed (usually through a cooler and a control valve) into the intake system downstream of the turbocharger compressor.
The temperature of gas recirculated in this way, in other words the “EGR gas”, can be controlled using a valve to selectively route part of the EGR gas through a cooler, with the remainder bypassing the cooler.
Furthermore, the temperature of the air-EGR charge mixture entering the engine can be controlled by using a valve to selectively route part of the air or air-EGR charge mixture through a charge cooler, with the remainder bypassing the cooler.
By using the arrangements illustrated in FIG. 1 and FIG. 2 and by controlling the balance of boosting work carried out by the turbocharger and the supercharger the capacity and range of the EGR system can be improved and fuel economy can be further optimized consistent with emissions requirements.
In particular the systems could deliver exhaust gas from upstream of the turbocharger turbine to position 1, 2 or 3. The systems could deliver exhaust gas from downstream of the turbocharger turbine to position 1. System la could deliver exhaust gas from downstream of the turbocharger turbine to position 2.
By using the supercharger in combination with a turbocharger and with a suitable control system it is also possible to arrange for the pressure at position 3 to be higher than the pressure upstream of the turbocharger turbine. In combination with suitable valve event timing this can be used to scavenge the cylinder of hot exhaust gas thus reducing combustion temperature and NOx emissions. Furthermore this scavenging effect can be used to improve volumetric efficiency and hence performance.
The forgoing is provided in order to assist the addressee in understanding possible applications of the superchargers described in WO-A1-2004/072449. However, it is envisaged that such superchargers may also be used singly, that is without a turbocharger, to boost the inlet pressure to an automotive engine.
It will be understood by the skilled addressee that many engine management systems for automotive engines that are arranged to operate with a supercharger and/or a turbocharger, rely on being able to control the pressure of the charge at the inlet to the engine to correspond to a predetermined required pressure for current, or desired, operating conditions. A range of such predetermined required engine inlet pressures may, for example, be stored in a look-up table to which the engine management system has access. Controlling the engine inlet pressure is very important for correct and efficient operation of the engine. In the arrangements shown in Figure X and Figure Y, this engine inlet pressure corresponds to the pressure at position 3; in the arrangement referred to above in which such a supercharger is used on its own, without a turbocharger in series therewith, this pressure corresponds to the pressure at the outlet of the supercharger compressor.
However, the rate at which the supercharger can respond to demand and add boost pressure to the intake system is relatively fast and is typically an order of magnitude faster than the rate at which a turbocharger can respond. This means that control of the supercharger based on actual versus target boost pressure is likely to result in instability and overshooting, and/or to unfavourable interaction with a turbocharger controller in the event that the supercharger is used with a turbocharger.
Embodiments of the present invention are directed towards providing control of the outlet pressure of a supercharger of the type described hereinabove in order to address this problem.